1. Field of the Invention
The present invention relates generally to the field of molecular biology. More specifically, the invention relates to plant genes involved in phosphorous uptake and methods of use thereof.
2. Description of the Related Art
Phosphate (Pi) is one of the key substrates in energy metabolism and biosynthesis of nucleic acids and membranes. It also plays an important role in photosynthesis, respiration and regulation of a number of enzymes (Raghothama, 1999). While it is a critical macronutrient for plant growth and development, most of the total soil phosphorus (P) is not available for uptake due to its rapid immobilization by soil organic and inorganic components (Von Uexküll and Mutert, 1995; Whitelaw, 2000). Phosphorus is limiting for crop yield on over 30% of the world's arable land, and by some estimates, world resources of inexpensive rock phosphate may be depleted by 2050 (Vance et al., 2003). The lack of inexpensive P has been recognized as a potential future crisis in agriculture (Abelson, 1999). In consideration of the trend toward sustainability and environmental stewardship, P has been a key nutrient in maintaining long-term productivity of agricultural systems (Iyamuremye and Dick, 1996).
The P cycle can be characterized as the flow of P between plants, animals, microorganisms and solid phases of the soil (Iyamuremye and Dick 1996). A significant proportion of the soil P is in organic forms, either as specific organic P compounds or as organic compounds to which inorganic P is linked (Larsen 1967; Bieleski 1973). Organic P generally makes up 20% to 80% of the total P in the surface layer of the soil, which, after mineralization, can contribute considerably to the P nutrition of plants (Dalal 1977; Iyamuremye and Dick 1996). The predominant form of organic P is phytate (inositol hexa- and penta-phosphates), which composes up to 60% of soil organic P and is poorly utilized by plants (Iyamuremye and Dick 1996; Mudge et al., 2003).
Phytate can be hydrolyzed to inorganic phosphate (Pi) and myo-inositol through the action of phytase enzymes (Mudge et al., 2003). In the study of phytases, much attention has been paid to the use of phytases as an animal feed additive, because phytate in plant seeds is largely indigestible by monogastric animals (reviewed by Wodzinski and Ullah 1996; Brinch-Pedersen et al., 2002; Vohra and Satyanarayana 2003). Phytases have been commercially produced based on the filamentous fungus Aspergillus niger (Brinch-Pedersen et al., 2002). By comparison, phytases in plant roots have received much less attention; the potential of producing phytase in plant roots for improved P uptake has only been recognized in recent years. Application of a fungal phytase to sterile cultures of subterranean clover (Trifolium subterraneum) enabled the seedlings to use phytate as the only source of P (Hayes et al., 2000). Ectopic expression of a fungal phytase gene (Richardson et al., 2001; Mudge et al., 2003) or a synthetic phytase gene (Zimmermann et al., 2003) resulted in increased P acquisition and biomass production in transgenic plants.
Phytases have been identified in roots of plants (Hübel and Beck 1996; Li et al., 1997; Hayes et al., 1999; Richardson et al., 2000) such as maize (Maugenest et al., 1997; Maugenest et al., 1999) and soybean (Hegeman and Grabau 2001). (Hübel and Beck 1996; Li et al., 1997; Hayes et al., 1999; Richardson et al., 2000). However, it has been suggested that the activity of these enzymes in roots is inadequate for effective utilization of organic P (Hayes et al., 1999; Richardson et al., 2000; Brinch-Pedersen et al., 2002). Additionally, in these plants phytases do not appear to be secreted or involved in P acquisition of roots from external phytate (Hübel and Beck 1996; Maugenest et al., 1999; Hegeman and Grabau 2001). To date, there have been no reports on improving P uptake by transgenically expressing any phytase genes of plant origin. There is, therefore, a great need in the art for new genes capable of improving P utilization.